This invention relates in general to optical communication systems, and in particular to a system and method for controlling spectral distribution of output power in Raman amplifiers.
Optical communication networks, in particular long-haul networks of lengths between 600 kilometers and 10,000 kilometers, inevitably suffer from signal attenuation due to variety of factors including scattering, absorption, and bending. To compensate for losses, optical amplifiers, regenerators, or a combination thereof are typically placed at regular intervals along the optical transmission path. Optical amplifiers amplify an input optical signal without converting it into electrical form. In contrast, regenerators convert optical signals into electrical form and then back to optical form in order to amplify, reshape, retime, and retransmit the optical signal.
Optical amplifiers include rare earth doped fibers such as erbium doped fiber amplifiers (EDFAs) and Raman amplifiers. An EDFA operates by passing an optical signal through an erbium-doped fiber segment, and xe2x80x9cpumpingxe2x80x9d the segment with light from another source such as a laser. Raman amplification occurs throughout an optical transmission fiber when the transmission fiber is pumped at an appropriate wavelength or wavelengths. Gain is then achieved at a longer wavelength through the process of Stimulated Raman Scattering. The difference between the Raman amplifier pumped wavelength and the associated amplified wavelength spectrum at the longer wavelength is referred to as a xe2x80x9cStokes shift.xe2x80x9d The Stokes shift for a typical silica fiber is approximately 13 THz. Hence, Raman amplifiers provide amplification of an optical signal without the need for a specially doped fiber, such as used in an EDFA.
Many optical communication systems, especially long-haul networks, are wavelength division multiplexed (WDM) or dense wavelength division multiplexed (DWDM) systems. Such systems incorporate a plurality of information channels carried by slightly different wavelengths. Accordingly, optical amplifiers in such systems should be capable of amplifying all the wavelengths at a relatively consistent gain level in a broad wavelength range.
A Raman amplifier with a single pump may fail to provide gain over the bandwidth required in some WDM and DWDM optical systems. To achieve a broadband gain characteristic, a plurality of pumps may be utilized in a single Raman amplifier. However, the gain spectrum from each pump tends to overlap such that a multi-pump Raman amplifier typically exhibits some gain variation over the spectral range of the amplifier, known as xe2x80x9cripple.xe2x80x9d This disparity in imparted gain can negatively effect signal detection and system reliability.
In addition, in many systems, the distance and losses between optical amplifiers varies over a wide range. This makes the input powers to the amplifiers different. Since the actual launched powers at the output of each amplifier must be carefully controlled, the actual gains over each amplifier are different. The preferred operating conditions of each amplifier is constant or controlled spectral distribution of output power.
Accordingly, there is a need for a Raman amplifier pump configuration and method that overcomes the deficiencies of the prior art by providing a desired spectral distribution of output power characteristic across a transmitted range of wavelengths in a Raman amplifier for all possible spectral distributions of the input power.
An amplifier consistent with the invention includes a plurality of pumps and a feedback control system. The pumps produce an amplifier gain characteristic. The control system is configured to receive an output signal representative of an output of the amplifier and provide at least one control signal to at least one of the pumps in response to the output signal. The control signal causes adjustment of at least one adjustable parameter of the pump to achieve a desired amplifier gain characteristic. An optical communication system consistent with the invention includes a transmitter for transmitting an optical signal on an optical information channel and an amplifier consistent with the invention coupled to the information channel.
A method of obtaining a desired gain characteristic for an optical amplifier including a plurality of pumps consistent with the invention includes: detecting an output signal representative of an output of the amplifier; generating at least one control signal in response to the output signal; and adjusting at least one adjustable parameter of at least one of the pumps in response to the control signal to achieve the desired amplifier spectral distribution of output power characteristic.